Frequency conversion through nonlinear optical processes has been ubiquitously used to generate laser light at spectral regions from UV to far infrared where direct laser light is not available. Ever since the first SHG by Franken et al. in 1961, it is recognized that an efficient frequency conversion depends critically on both phase matching and pump light intensity. Until now, birefringent PM and quasi-phase matching (QPM) are two main techniques successfully used. The former relies on incidental PM between the pump and the harmonics having different polarizations. The latter typically requires artificial patterning of the optical nonlinearity of the material. The most commonly used patterning method is periodic poling which unfortunately excludes most nonlinear crystals. Recent research efforts have explored various PM conditions such as modal dispersion, form birefringence and random PM.
Optical whispering gallery mode (WGM) resonators made of crystalline materials are capable of ultra-high quality (Q) factors and confine light in small mode volumes. See for example V. S. Ilchenko, A. B. Matsko, and L. Maleki, “Kilohertz optical resonances in dielectric crystal cavities,” Phys. Rev. A 70, 051804 (2004). These features have drawn strong interest in nonlinear optics applications. See for example, A. A. Savchenkov, A. B. Matsko, V. S. Ilchenko, I. Solomatine, D. Seidel, and L. Maleki, “Tunable optical frequency comb with a crystalline whispering gallery mode resonator,” Phys. Rev. Lett. 101, 93902 (2008); V. S. Ilchenko, A. A. Savchenkov, A. B. Matsko and L. Maleki, “Nonlinear optics and crystalline whispering gallery mode cavities,” Phys. Rev. Lett. 92, 43903 (2004); and J. U. Furst, D. V. Strekalov, D. Elser, M. Lassen, U. L. Andersen, C. Marquardt and G. Leuchs, “Naturally phase-matched second-harmonic generation in a whispering-gallery-mode resonator,” Phys. Rev. Lett. 104, 153901 (2010).
For facile phase matching of these nonlinear processes, birefringent crystals are used. The resonator symmetry axis is commonly chosen to coincide with the crystal optic axis, which are referred to as hence z-cut resonators. Natural phase matching condition can be achieved in the z-cut crystals with a narrow wavelength range. Periodic poling in WGM resonators of ferroelectric crystals has also been studied for quasi phase matching. Most of commonly used ferroelectric crystals are not suited at UV wavelength due to their larger optical absorption. Beta barium borate (BBO) is an important nonlinear crystal used in visible and UV wavelength range. It is a uniaxial crystal with a large birefringence and wide transparency range from 198 to 2100 nm.
The growth of beta barium borate has been reported in the literature and methods of growth of BBO crystals have been patented. See for example, Gualtieri et al., U.S. Pat. No. 4,931,133, issued Jun. 5, 1990, which is said to disclose a method of growing optical quality β-BaB2O4 crystals that involves growing these crystals from a fluxed melt of BaB2O4, Na2O and NaCl, in certain proportions.
Optical frequency conversion processes in nonlinear materials are limited by the accessible phase matching conditions and require high pump powers.
There is a need for systems and methods that provide convenient phase matching and second harmonic generation over a wide range of wavelengths.